The high specificity of biological interactions is fundamental to the organization of living systems. An important approach to understanding specificity at a molecular level is study of structure for which there is crystallographic information at atomic resolution. Such studies are expected to lead to generalizations that sharpen our present concepts. Towards this end, the chemical properties and crystal structures of lactate and malate dehydrogenases will be correlated in order to define details of the ligand-binding reactions, and also the fluorescence properties of these molecules.